docs: Flesh out token swap README

[joncinque]

Now that the token swap program is out in the wild, it's really time for some documentation.

The idea is to follow the docs at token, specifically focusing on:

• How the accounts are created and used. Devs (myself included) have initial struggles understanding the many accounts required by Solana programs, so the idea is to also provide background information about accounts
• Curves present. If people want to implement their own curve, they should know that it's possible and what's already in the program.

[bartosz-lipinski]

Looks awesome @joncinque I made some edits

Pull request has been modified.